The present invention relates to display apparatuses. The display apparatus may be a directional display apparatus such as a switchable two dimensional (2D)/three dimensional (3D) autostereoscopic display apparatus; a switchable high brightness reflective display apparatus; or a multi-user display apparatus. Such display apparatuses may be used in computer monitors, telecommunications handsets, digital cameras, laptop and desktop computers, games apparatuses, automotive and other mobile display applications.
In particular, the present invention relates to the alignment of elements of a display apparatus, that is the alignment of a display substrate comprising an array of pixels with a component substrate comprising an array of optical components, such as lenses.
Normal human vision is stereoscopic, that is each eye sees a slightly different image of the world. The brain fuses the two images (referred to as the stereo pair) to give the sensation of depth. Three dimensional stereoscopic displays replay a separate, generally planar, image to each of the eyes corresponding to that which would be seen if viewing a real world scene. The brain again fuses the stereo pair to give the appearance of depth in the image.
FIG. 1a shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 3 on the display plane and a left eye 4 views a left eye homologous point 5 on the display plane to produce an apparent image point 6 perceived by the user behind the screen plane.
FIG. 1b shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 7 on the display plane and a left eye 4 views a left eye homologous point 8 on the display plane to produce an apparent image point 9 in front of the screen plane.
FIG. 1c shows the appearance of the left eye image 10 and right eye image 11. The homologous point 5 in the left eye image 10 is positioned on a reference line 12. The corresponding homologous point 3 in the right eye image 11 is at a different relative position 3 with respect to the reference line 12. The separation 13 of the point 3 from the reference line 12 is called the disparity and in this case is a positive disparity for points which will lie behind the screen plane.
For a generalised point in the scene there is a corresponding point in each image of the stereo pair as shown in FIG. 1a. These points are termed the homologous points. The relative separation of the homologous points between the two images is termed the disparity; points with zero disparity correspond to points at the depth plane of the display. FIG. 1b shows that points with uncrossed disparity appear behind the display and FIG. 1c shows that points with crossed disparity appear in front of the display. The magnitude of the separation of the homologous points, the distance to the observer, and the observer's interocular separation gives the amount of depth perceived on the display.
Stereoscopic type displays are well known in the prior art and refer to displays in which some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be colour filters in which the images are colour coded (e.g. red and green); polarising glasses in which the images are encoded in orthogonal polarisation states; or shutter glasses in which the views are encoded as a temporal sequence of images in synchronisation with the opening of the shutters of the glasses.
Autostereoscopic displays operate without viewing aids worn by the observer. In autostereoscopic displays, each of the views can be seen from a limited region in space as illustrated in FIG. 2.
FIG. 2a shows a display device 16 with an attached parallax optical element 17. The display device produces a right eye image 18 for the right eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 19 to produce a right eye viewing window 20 in the region in front of the display. An observer places their right eye 22 at the position of the window 20. The position of the left eye viewing window 24 is shown for reference. The viewing window 20 may also be referred to as a vertically extended optical pupil.
FIG. 2b shows the left eye optical system. The display device 16 produces a left eye image 26 for the left eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 28 to produce a left eye viewing window 30 in the region in front of the display. An observer places their left eye 32 at the position of the window 30. The position of the right eye viewing window 20 is shown for reference.
The system comprises a display and an optical steering mechanism. The light from the left image 26 is sent to a limited region in front of the display, referred to as the viewing window 30. If an eye 32 is placed at the position of the viewing window 30 then the observer sees the appropriate image 26 across the whole of the display 16. Similarly the optical system sends the light intended for the right image 18 to a separate window 20. If the observer places their right eye 22 in that window then the right eye image will be seen across the whole of the display. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution.
FIG. 3 shows in plan view a display device 16,17 in a display plane 34 producing the left eye viewing windows 36,37,38 and right eye viewing windows 39,40,41 in the window plane 42. The separation of the window plane from the display is termed the nominal viewing distance 43. The windows 37,40 in the central position with respect to the display are in the zeroth lobe 44. Windows 36,39 to the right of the zeroth lobe 44 are in the +1 lobe 46, while windows 38,41 to the left of the zeroth lobe are in the −1 lobe 48.
The viewing window plane of the display represents the distance from the display at which the lateral viewing freedom is greatest. For points away from the window plane, there is a diamond shaped autostereoscopic viewing zone, as illustrated in plan view in FIG. 3. As can be seen, the light from each of the points across the display is beamed in a cone of finite width to the viewing windows. The width of the cone may be defined as the angular width.
If an eye is placed in each of a pair viewing zones such as 37,40 then an autostereoscopic image will be seen across the whole area of the display. To a first order, the longitudinal viewing freedom of the display is determined by the length of these viewing zones.
The variation in intensity 50 across the window plane of a display (constituting one tangible form of a directional distribution of the light) is shown with respect to position 51 for idealised windows in FIG. 4a. The right eye window position intensity distribution 52 corresponds to the window 41 in FIG. 3, and intensity distribution 53 corresponds to the window 37, intensity distribution 54 corresponds to the window 40 and intensity distribution 55 corresponds to the window 36.
FIG. 4b shows the intensity distribution with position schematically for more realistic windows. The right eye window position intensity distribution 56 corresponds to the window 41 in FIG. 3, and intensity distribution 57 corresponds to the window 37, intensity distribution 58 corresponds to the window 40 and intensity distribution 59 corresponds to the window 36.
The quality of the separation of images and the extent of the lateral and longitudinal viewing freedom of the display is determined by the window quality, as illustrated in FIG. 4. FIG. 4a shows the ideal viewing windows while FIG. 4b is a schematic of the actual viewing windows that may be outputted from the display. Several artefacts can occur due to inadequate window performance. Cross talk occurs when light from the right eye image is seen by the left eye and vice versa. This is a significant 3D image degradation mechanism which can lead to visual strain for the user. Additionally, poor window quality will lead to a reduction in the viewing freedom of the observer. The optical system is designed to optimised the performance of the viewing windows.
The parallax element may be a parallax barrier comprising an array of opaque regions alternating with transmissive regions. Parallax barriers rely on blocking the light from regions of the display and therefore reduce the brightness and device efficiency, generally to approximately 20-40% of the original display brightness. Parallax barriers are not readily removed and replaced due to the requirements of sub-pixel alignment tolerances of the barrier with respect to the pixel structure of the display in order to optimise the viewing freedom of the display. The 2D mode is half resolution.
Another type of parallax optic alternative to a parallax barrier well known in the art for use in stereoscopic displays is a lenticular screen, which is an array of vertically extended cylindrical microlenses.
FIG. 5 shows a typical structure of a prior art display device using a lenticular screen. A backlight 60 produces a light output 62 which is incident on an LCD input polariser 64. The light is transmitted through a TFT LCD substrate 66 and is incident on a repeating array of pixels arranged in columns and rows in an LCD pixel plane 67. The red pixels 68,71,73, green pixels 69,72,75 and blue pixels 70,73 each comprise an individually controllable liquid crystal layer and are separated by regions of an opaque mask called a black mask 76. Each pixel comprises a transmissive region, or pixel aperture 78. Light passing through the pixel is modulated in phase by the liquid crystal material in the LCD pixel plane 74 and in colour by a colour filter positioned on an LCD colour filter substrate 80. The light then passes through an output polariser 82 after which is placed a lenticular screen substrate 94 and a lenticular screen 96 which is formed on the surface of the lenticular screen substrate 92. As for the parallax barrier, the lenticular screen 94 serves to direct light from alternate pixel columns 69,71,73,75 to the right eye as shown by the ray 88 from the pixel 69 and from the intermediate columns 68,70,72,74 to the left eye as shown by the ray 90 from pixel 68. The observer sees the light from the underlying pixel illuminating the aperture of the individual lenticule, 98 of the lenticular screen 96. The extent of the captured light cone is shown by the captured rays 100.
In the above arrangement, the LCD pixel plane acts as a spatial light modulator (SLM). As used in this document, the term spatial light modulator or SLM includes both ‘light valve’ devices such as liquid crystal displays and emissive devices such as electroluminescent displays and LED displays.
As described above, the use of parallax optics to generate a spatially multiplexed 3D display limits the resolution of each image to at best half of the full display resolution. In many applications, the display is intended to be used for a fraction of the time in the 3D mode, and is required to have a full resolution artefact free 2D mode.
An example of a switchable 2D-3D display is described in is shown in plan view in FIG. 6. A backlight 102 produces illumination 104 of an LCD input polariser 106. The light passes through a thin film transistor (TFT) substrate 108 and is incident on a pixel layer 110 comprising individually controllable phase modulating pixels 112-126. The pixels are arranged in rows and columns and comprise a pixel aperture 128 and a separating black mask 130. The light then passes through an LCD counter substrate 132 and a lens carrier substrate 136 upon which is formed a birefringent microlens array 138. The birefringent microlens array 138 comprises an isotropic lens microstructure 140 and an aligned birefringent material with an optical axis direction 142. The output of the birefringent lens then passes through a lens substrate 144 and a polarisation modifying device 146.
Each birefringent lens of the lens array is cylindrical; the lens array 138 is a lenticular screen and the geometrical axis of the lenses is out of the page. The pitch of the lenses in this example is arranged to be substantially twice the pitch of the pixels of the display such that a two view autostereoscopic display is produced.
In a first mode of operation, the polarisation modifying device 146 is configured to transmit light with a polarisation state which is parallel to the ordinary axis of the birefringent material of the microlens array. The ordinary refractive index of the material (such as a liquid crystal material) is substantially matched to the index of the isotropic microstructure 140. Thus the lenses have no optical effect and there is substantially no change to the directional distribution of the output of the display. In this mode, an observer will see all the pixels 112-126 of the display with each eye, and a 2D image will be produced.
In a second mode of operation, the polarisation modifying device 146 is configured to transmit light with a polarisation state which is parallel to the extra-ordinary axis of the birefringent microlens array. The extraordinary refractive index of the material (such as a liquid crystal material) is different to the index of the isotropic microstructure 140. Thus the lenses have an optical effect and there is a change to the directional distribution of the output of the display. This directional distribution can be set as well known in the art so as an observer correctly positioned at the front of the display will see a left image in their left eye corresponding to light from left image pixels 112,116,120,124 and in their right eye will see a right image corresponding to right image pixels 114,118,122,126. In this way, a switchable 2D to 3D autostereoscopic display can be produced.
Lens arrays are particularly suitable for autostereoscopic displays because they combine the functionalities of high efficiency, small spot size and ability to manufacture using well known lithographic processing techniques.
It is known to provide electrically switchable birefringent lenses for purposes of switching light directionally. It is known to use such lenses to switch a display between a 2D mode of operation and a 3D mode of operation.
In a 3D autostereoscopic display, a lens array (or lenticular screen) may be used to direct the light into a directional distribution consisting of the appropriate viewing windows. This type of lens array may also be used to direct light into other directional distributions. Examples of such directional distributions are disclosed in WO-03/015,424 and include an enhanced brightness distribution in which light is directed into broad horizontal windows in the nominal viewing plane. In the window, the observer will see an increase of brightness proportional to the vertical aperture ratio of the panel. Out of the viewing window, an observer will see the gaps between the pixels, and the display has reduced brightness.
FIG. 8 shows the relative alignment of the lenses of an autostereoscopic two view display to the pixels in order to achieve the required alignment at various points across the display surface. For simplicity of explanation, the lenses are shown in overhead view, while the pixels are marked with their relative alignment to the lenses in plan view. For simplicity of illustration, only selected panel pixels are shown.
In an ideally aligned two view 3D autostereoscopic display, light rays 300 from the centre of the gap between the right eye pixel columns 304 and left eye pixel columns 302 is directed by a first adjacent lens 312 to the window centre as shown in FIG. 8. In order to achieve this, the position of the lens geometric axis is directly over the centre of the pixel gap for the central lens 306, while for lenses at the edge of the display 308, 310, the geometric lens axis position 318 is towards the centre of the display from the centre of the pixel gap 314. Such an alignment condition is known as viewpoint correction.
In manufacture, alignment tolerances of the devices must be met so that the display can be comfortably viewed. In particular, the lateral lens optical axis position is preferably aligned to the respective pixel aperture to within less than for example 5% of the pixel size.
If the window centre is not aligned on the centre line of the display, then the viewer may experience discomfort when viewing the stereo image, and may additionally find the best viewing position is off-centre of the display. For many display types, such as for example those with colour sub-pixel sizes in the region 40-1001 μm in horizontal pitch this requires an alignment tolerance of the geometric lens axis to the panel pixel layout of 2-5 μm across the display area, and is similar to the alignment tolerances of colour filter substrates to TFT substrates.
If the geometric lens axis 306 is not parallel to the pixel vertical axis 316, then the window centre will vary down the height of the panel. This will result in a reduction of viewing freedom of the panel which is highly undesirable.
Other autostereoscopic display systems well known in the art use arrays of other types of optical component for example switchable liquid crystal lens arrays, fixed lens arrays, prism arrays, aperture arrays (known as parallax barriers) and hologram arrays. All of these devices require similar alignment tolerances of the micro-optic component to the display device pixels.
Displays showing more than two views including those in which cylindrical lenses are tilted with respect to the pixel columns are also known in the art. In this case, it is desirable that the viewing window centre is aligned with the display centre line so that the viewing freedom is symmetric about a central viewing position.